I think there's some argument for this, but I don't think it's entirely because Nicki is just so completely perfect for Guy. Instead, I think it's the same problem that I always have playing RPGs. Usually you start out, and you get an NPC in your party who's closely related to you -- a childhood friend, a close peer, whatever. And even though I have the option to swap them out, I never do because.. C'mon. That's my dude! They've been there since the beginning, I can't just change them out now.
I think Nicki just seems like the best/only real option, due to the fact she's been around the longest. But, I think that year-long gap (and Nicki's initial reluctance to date) provides enough cover for Guy to look seriously at other women, and agree that he and Nicki are better off just being friends. Sometimes you've got the right person, but stuff just doesn't line up. It sucks, but it happens.
Ultimately, I think the choice of love interest just depends on what type of relationship you want to have. A character like Carolina challenges Guy to step up and step into the role of pseudo-dad, and maybe (eventually) actual step-dad, which I think could be appealing for some people. Brittani and Gabby are younger, and give Guy the opportunity to relive his wasted 20s, while also trying to shepherd them through life's rough spots. Risa presents the option for a very free-love, zero responsibility, open relationship.
If someone's only interest is in a grand, fated sort of romance.. Nicki might be the best choice! But other people may want other things. Including just being kind of a degenerate with a harem of sugarbabies. >_>